red supergiant

More than two dozen stars, appearing yellow near
the center of this Spitzer
Space Telescope image of the cluster RSGC2 are red supergiants.
Warm dust in the region glows red.
The blue oval with a pink outline at top left may be the result of
an ancient supernova and the larger blue patch below center is a region
of current star formation.
Image credit: B. Davies/RIT/NASA.

Red supergiant by Chesley Bonestell.

A red supergiant is a star that is similar in nature but bigger and more massive than a red
giant. Red supergiants typically span several hundred times the diameter
of the Sun and have masses of more than 10 times that of the Sun. They belong
to luminosity class I and are of spectral typeM or K.

The red supergiant phase is extremely short-lived, lasting only a few hundred
thousand to a million years. at most before ending in a supernova (see stars, evolution). It is believed
that high-mass red supergiants evolve to become Wolf-Rayet stars before
exploding as type Ib or Ic supernovae, while those of lower mass eventually
explode as type II supernovae.

Due to the scarcity of very massive stars and the briefness of the red supergiant
stage, this type of star is extremely rare. Only about 200 are known in
the entire Galaxy. Well-known examples include Betelgeuse and Antares.
Among the largest known supergiants is Mu Cephei, also known as the Garnet
Star.

Clusters of red supergiants

Although red supergiants are rare, clusters of them do exist. In 2006, a
team led by Don Figer of the Rochester Institute of Technology (RIT) in
New York announced it had found a massive cluster of about 20,000 stars,
called RSGC1, which included 14 red supergiants. Lying near the center of
the Galaxy, it was the largest grouping of these stars then known.

In 2007, Ben Davies, also of RIT, and a team that includes Figer reported
the discovery of an even bigger collection of 26 red supergiants in another
cluster of young stars called RSGC2, which lies just a few hundred light-years
from RSGC1. The two clusters are similar in age – both less than 20
million years old – although RSG2 contains more than twice as many
stars, making it one of the most massive clusters of young stars in the
Galaxy. The nearness of RSG1 and RSG2 is almost certainly not coincidental.
There is other evidence of star formation in this region, which is where
the Scutum-Crux spiral arm of the Galaxy meets the central bulge.